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Internet: http://planetquest1.jpl.nasa.gov/venus_transit.cfm →
On June 8, 2004, Venus crossed the Sun's disc for about seven hours, an event not seen since 1882. The transit demonstrated a key technique being used to detect planets orbiting stars outside our solar system. Just as Venus temporarily dimmed the light of our star -- the Sun -- as it crossed in front of it, a distant planet crossed over and blocked the light of its parent star. Several planned missions will hunt for such "extrasolar" planets as part of NASA's Origins program, which seeks to answer the questions: Where did we come from? Are we alone?